RFC 4867, "RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs", April 2007Source of RFC: avt (rai)
See Also: RFC 4867w/ inline errata
Errata ID: 4348
Publication Format(s) : TEXT
Reported By: Thomas Belling
Date Reported: 2015-04-27
Verifier Name: Ben Campbell
Date Verified: 2016-05-25
Section 4.3.1 says:
The encoder SHOULD follow a received codec mode request, but MAY change to a lower-numbered mode if it so chooses, for example, to control congestion.
It should say:
The encoder MUST follow a received codec mode request as soon as possible. It SHOULD use the requested mode, but MAY change to a lower-numbered mode if it so chooses, for example, to control congestion. However, the encoder MUST NOT use a higher-numbered mode than the received codec mode request.
This seems to be the intention of the existing text, but is not explicitly stated. It is essential for the end-to-end mode control and interoperability with 3GPP CS networks that a peer does not send higher modes than requested.
This erratum has been discussed and endorsed by the 3GPP SA4 group before submission to IETF.